The methods for producing organic light-emitting devices (LEDs) broadly fall into two categories, vacuum vapor deposition and coating. The coating method has advantages of easily producing large-area films and high material utilization factor. In order to apply the coating method, development of devices with a single light-emitting layer is demanded, because it is necessary to reduce number of organic LED layers.
For organic white-light-emitting devices with a single light-emitting layer, Patent Document 1 discloses an organic EL device with a single light-emitting layer of a composition of at least (a) polymer and (b) compound for forming a light-emitting center, placed between electrodes. The composition contains an electron transfer material and hole transfer material in a well-balanced manner. The polymer itself emits blue or shorter wavelength colors, and is dispersed with at least two species of compounds for forming a light-emitting center, each individually emitting a color, where a combination of these compounds is selected in such a way that the organic EL device as a whole emits white color.
Recently, organic light-emitting devices have been attracting attention as planar light sources of the next generation. An organic light-emitting device exhibits excellent characteristics of spontaneously emitted light, wide view angle, high responsiveness and high color rendition. One example of organic light-emitting device comprises a glass substrate which supports a transparent electrode (e.g., of ITO), organic layer composed of hole transport layer, light-emitting layer, electron transport layer and so on, and repeller of low work function, where light is emitted from the back side of the substrate after passing through the electrode.
The methods for producing organic light-emitting devices include vacuum vapor deposition, ink jetting and printing. Vacuum vapor deposition heats and evaporates an organic material under a vacuum and deposits the vapor on a substrate to form the film thereon. It can simply produce an organic light-emitting device of desired structure because of its capability of controlling film thickness and concentration by manipulating deposition rate. However, it involves disadvantages of low material utilization factor and difficulty in increasing substrate size. Ink jetting or printing is expected to be a low-cost wet method, because of its advantages of high material utilization factor and easily forming large-area device, although involving a disadvantage of difficulty in forming laminated structures because solubility of an underlying material must be controlled for forming a laminated structure.
An organic light-emitting device must emit white color when used as a light source. It is necessary for the device to emit three colors of red, blue and green in order to realize white color of high rendition.
The wet methods proposed so far for producing organic white-light-emitting devices include mixing three species of dopants each emitting red, blue or green color in a light-emitting layer. For example, Patent Document 1 discloses a method which uses polyvinyl carbazole as a host material for light-emitting layer which is dispersed with red-, blue- and green-light-emitting dopants at a low concentration (0.01 to 5% by mol) to realize white color.    [Patent Document 1] JP-A-9-63770